This commit aims to not make use of -Wmissing-prototypes when
compiling with g++.

Use of -Wmissing-prototypes was added with this commit:

  commit a0761e34
  Date:   Wed Mar 11 15:15:12 2020 -0400

      gdb: enable -Wmissing-prototypes warning

Because clang can provide helpful warnings with this flag.
Unfortunately, g++ doesn't accept this flag, and will give this
warning:

  cc1plus: warning: command line option ‘-Wmissing-prototypes’ is valid for C/ObjC but not for C++

In theory the fact that this flag is not supported should be detected
by the configure check in gdbsupport/warning.m4, but for users of
ccache, this check doesn't work due to a long standing ccache issue:

  https://github.com/ccache/ccache/issues/738

The ccache problem is that -W... options are reordered on the command
line, and so -Wmissing-prototypes is seen before -Werror.  Usually
this doesn't matter, but the above warning (about the flag not being
valid) is issued before the -Werror flag is processed, and so is not
fatal.

There have been two previous attempts to fix this that I'm aware of.
The first is:

  https://sourceware.org/pipermail/gdb-patches/2021-September/182148.html

In this attempt, instead of just relying on a compile to check if a
flag is valid, the proposal was to both compile and link.  As linking
doesn't go through ccache, we don't suffer from the argument
reordering problem, and the link phase will correctly fail when using
-Wmissing-prototypes with g++.  The configure script will then disable
the use of this flag.

This approach was rejected, and the suggestion was to only add the
-Wmissing-prototypes flag if we are compiling with gcc.

The second attempt, attempts this approach, and can be found here:

  https://sourceware.org/pipermail/gdb-patches/2021-November/183076.html

This attempt only adds the -Wmissing-prototypes flag is the value of
GCC is not 'yes'.  This feels like it is doing the right thing,
unfortunately, the GCC flag is really a 'is gcc like' flag, not a
strict, is gcc check.  As such, GCC is set to 'yes' for clang, which
would mean the flag was not included for clang or gcc.  The entire
point of the original commit was to add this flag for clang, so
clearly the second attempt is not sufficient either.

In this new attempt I have added gdbsupport/compiler-type.m4, this
file defines AM_GDB_COMPILER_TYPE.  This macro sets the variable
GDB_COMPILER_TYPE to either 'gcc', 'clang', or 'unknown'.  In future
the list of values might be extended to cover other compilers, if this
is ever useful.

I've then modified gdbsupport/warning.m4 to only add the problematic
-Wmissing-prototypes flag if GDB_COMPILER_TYPE is not 'gcc'.

I've tested this with both gcc and clang and see the expected results,
gcc no longer attempts to use the -Wmissing-prototypes flag, while
clang continues to use it.

When compiling using ccache, I am no longer seeing the warning.